For example, vehicles such as trucks, buses, and construction machines use compressed air delivered from the compressor, which is directly connected to the internal combustion engine, in order to control systems such as brakes and suspensions. In the system mentioned above and other systems, compressed air delivered from a compressor includes oil mist, which is atomized lubricant, as well as water contained in the atmosphere. When compressed air containing water and oil mist enters such a system, it may become a cause of an operation defect of the systems. Thus, an air dryer is provided downstream of the compressor to remove water and oil mist from compressed air (for example, Patent Document 1).
When executing a loading mode operation (dehumidification), the above described air dryer removes oil and water from compressed air. When executing an unloading mode (regeneration), the air dryer removes adsorbed oil and water from the desiccant and drains resultant liquid containing the removed oil and water to the outside. The air dryer delivers the drained liquid to an oil separator to prevent the liquid from being discharged onto the road surface. The oil separator causes air containing oil and water to strike impingement members to separate gas and liquid from each other, thereby collecting oil and discharging cleaned air.